The present disclosure relates to an engine exhaust structure.
Japanese Unexamined Patent Publication No. 2006-329031 describes an exhaust structure for a turbocharged engine. In this exhaust structure, an expanded portion with an increased diameter is provided directly downstream of a turbine. A straightening vane, which straightens a swirl flow discharged from the turbine, is provided inside the expanded portion. An exhaust gas purifier with a circular transverse section is connected downstream of the expanded portion. The exhaust gas purifier houses a catalytic converter. In this exhaust structure, the expanded portion straightens the flow of the exhaust gas, thereby uniformizing the velocity distribution of the exhaust gas passing through the exhaust gas purifier. As a result, catalyst exhibits sufficient purification performance.
If an exhaust gas purifier is connected directly downstream of a turbine, the exhaust gas purifier is close to an engine. In view of the layout inside an engine compartment, a flat exhaust gas purifier is conceivable. In the exhaust gas purifier, for example, a case housing a catalytic converter has a flat transverse section including a pair of facing short sides and a pair of facing long sides. This configuration efficiently places the exhaust gas purifier near the engine in a small engine compartment.
However, in the case where the exhaust gas purifier is connected directly downstream of the turbine, the exhaust gas flowing into the exhaust gas purifier becomes a strong swirl flow at a high turbine speed. If a strong swirl flow flows into a flat exhaust gas purifier, the velocity of exhaust gas is higher on the long sides than on the short sides in the exhaust gas purifier. After various studies, the present inventors found this fact. If the exhaust gas passing through the exhaust gas purifier has a biased velocity distribution, exhaust gas purification performance may deteriorate. An increase in the flow rate of the exhaust gas in a certain portion of the exhaust gas purifier may cause an excessive temperature rise at the certain portion, which leads to a heat damage.